Insulation materials are known which comprise a clean, non-toxic, heat barrier made of aluminum foil bonded to a single or double layer of polyethylene-formed bubbles spaced one bubble from another bubble in the so-called "bubble-pack" arrangement. Such non-foil bubble-packs are used extensively as packaging material, whereas the metal foil bubble-pack is used as thermal insulation in wood frame structures, walls, attics, crawl spaces, basements and the like and as wrapping for hot water heaters, hot and cold water pipes, air ducts and the like. The reflective surface of the metal, particularly, aluminum foil enhances the thermal insulation of the air-containing bubble pack.
Organic polymers, such as polyethylene, are generally considered to be high- heat-release materials. They can easily initiate or propagate fires because, on exposure to heat, they undergo thermal degradation to volatile combustible products. If the concentration of the degradation products in the air is within flammability limits, they can ignite either spontaneously, if their temperature is large enough, or by the effect of an ignition source such as a spark or flame. The ignition of polyethylene can be delayed and/or the rate of its combustion decreased by means of fire retardant materials.
The ultimate aim of fire retardants is to reduce the heat transferred to the polymer below its limit for self-sustained combustion or below the critical level for flame stability. This can be achieved by decreasing the rate of chemical and/or physical processes taking place in one or more of the steps of the burning process. One or a combination of the following can achieve fire extinguishing:
1. creation of a heat sink by using a compound that decomposes in a highly endothermic reaction giving non-combustible volatile products, which perform a blanketing action in the flame, e.g., aluminum or magnesium hydroxide; PA1 2. enhancements of loss of heat and material from the surface of the burning polymer by melt dripping, e.g., mixture of halogenated compounds with free radical initiators; PA1 3. flame poisoning by evolution of chemical species that scavenge H and OH radicals which are the most active in propagating thermo-oxidation in the flame, e.g., hydrogen halides, metal halides, phosphorus-containing moieties; PA1 4. limitation of heat and mass transfer across the phase boundary, between thermal oxidation and thermal degradation by creation of an insulating charred layer on the surface of the burning polymer, e.g., intumescent chart; or PA1 5. modification of the rate of thermal volatilization of the polymer to decrease the flammability of the volatile products; which approach strongly depends on the chemical nature of the polymer.
Fire retardant materials are generally introduced to the polyethylene as merely additives or as chemicals that will permanently modify its molecular structure. The additive approach is more commonly used because it is more flexible and of general application.
Generally, low density polyethylene films of 1-12 mil, optionally, with various amounts of linear low density polyethylene in admixture when additional strength is required, are used for the above applications. The insulating properties of the bubble pack primarily arise from the air in the voids. Typically, bubble diameters of 1.25 cm, 0.60 cm and 0.45 cm are present.
Regardless of the application method of fire retardant material(s), a satisfactory insulative assembly must have a fire rating of Class A with a flame spread index lower than 16, and a smoke development number smaller than 23. Further, the bonding of the organic polymer films and their aging characteristics must meet the aforesaid acceptable standards. Yet further, the fabrication method(s) of a new fire retardant system or assembly should be similar to the existing technology with reasonable and cost effective modifications to the existing fabrication system/technology. Still yet further, other physical properties of an improved fire standard system must at least meet, for example, the standard mechanical properties for duct materials as seen by existing competitive products.
Fire retardant polyethylene films, wires and cables containing a fire retardant material in admixture with the polyethylene per se are known which satisfy cost criteria and vigorous fire retardant technical standards to be commercially acceptable. However, it has been found that forming a bubble pack comprising such a film results in a poor bonding between the cavity-containing layer and the adjacent sealing layer used to cover the cavities to form the bubbles. Delamination of these layers, particularly, after installation constitutes a significant problem.
Conventional fire retardant additives are usually compounds of small molecular weights containing phosphorus, antimony, or halogens. The most effective commercially available fire retardant systems are based on halogen-containing compounds. However, due to concerns over the environmental effects of such halogenated compounds, there is an international demand to control the use of such halogenated additives.
Some of the most common halogenated agents are methyl bromide, methyl iodide, bromochlorodifluoromethane, dibromotetrafluoroethane, dibromodifluoromethane and carbon tetrachloride. These halogenated fire retarding materials are usually available commercially in the form of gases or liquids. Unlike chlorine and bromine, fluorine reduces the toxicity of the material and imparts stability to the compound. However, chlorine and bromine have a higher degree of fire extinguishing effectiveness and, accordingly, a combination of fluorine and either chlorine or bromine is usually chosen to obtain an effective fire-retarding compounds.
Other commercially available fire retardant materials that do not include halogens include boric acid and borate based compounds, monoammonium phosphonate, and urea-potassium bicarbonate.
Intumescent compounds which limit the heat and mass transfer by creating an insulating charred layer on the surface of the burning polymer are also considered fire retardant materials. A typical intumescent additive is a mixture of ammonium polyphosphate and pentaerythritol.
Fire retardant additives are often used with organic polymer/resins. Typically, a brominated or chlorinated organic compound is added to the polymer in admixture with a metal oxide such as antimony oxide. Halogenated compounds are also sometimes introduced into the polymer chain by co-polymerization. Low levels i.e. less than 1% W/W are recommended to make adverse effects of halogen-based systems negligible. Another common fire retardant additive is diglycidyl ether of bisphenol-A with MoO.sub.3. Other additives to improve the fire retarding properties of polyethylene include, for example, beta-cyclodextrin, magnesium hydroxide and alumina trihydrate, tin oxide, zinc hydroxystannate, and chlorosulphonated polyethylene.
A problem found with commercially available fire retardant polyethylene films, i.e. films comprising a fire retardant dispersed throughout the body of the film is that the fire retardant, generally, selected from the oxides of antimony, alumina trihydrate or magnesium hydroxide tend to migrate to and leach out of the film surface during the service life of the film and this constitutes an unsatisfactory aging characteristic. Further, because of the presence of the fire retardant at or adjacent to the film surface, heat sealing of multiple films together is also unsatisfactory. The unfavourable aging and heat sealing characteristics are a function proportional to the amount of additive in the film.
There is, therefore, a need for a thermal insulation system having improved fire retardant properties.